This invention relates to laser scanning flaw detection systems for the detection of flaws on a moving web of material, and more particularly to such a system in which a plurality of laser sources are simultaneously scanned across a moving web of material being inspected employing a common scanning means.
U.S. Pat. Nos. 3,900,265 entitled "Laser Scanner Flaw Detection System" and 3,980,891 entitled "Method and Apparatus for a Rotary Scanner Flaw Detection System", which patents are assigned to the assignee of the present invention, are illustrative of the type of system to which the present invention relates. In the aforesaid systems, flaws are detected in the material being examined by repetitively scanning a source of radiation in the form of a laser beam across the surface of material. The laser radiation is reflected, transmitted or scattered from the material depending upon the characteristics of the material being inspected. The laser radiation is picked up from the material by a receiver having suitable detectors such as photomultiplier tubes therein. At any instant in time during the scan, the photomultiplier output varies with the reflectivity, transmissivity or scattering properties of the spot on the material upon which the laser beam is impinging. Deviations in the signal from a normal characteristic signal provide a means for indicating that flaws exist in the material.
Another rotary scanner flaw detection system in U.S. Pat. No. 3,866,054 entitled "Defect Size Discriminator Circuit for Web Inspection System" illustrates the use of a receiver for picking up the radiation from the scanned web comprising a radiation conducting rod which conveys laser beam transmitted or reflected radiation from the material to a photomultiplier tube positioned on the end of the rod. A diffusing stripe is positioned in the rod so that when the radiation is applied thereto from the material being inspected, the radiation is dispersed within the rod causing internal reflection therein, thereby transmitting the radiation through the rod to the photomultiplier tube positioned on the end thereof.
In the aforesaid systems after signals have been generated in accordance with the intensity of radiation applied to the detectors from the material being examined, such signals are processed in electronic processing circuitry to identify the flaws in the material as well as provide other information with respect to such flaws such as their position on the material, the relative size of the flaws, whether the same flaws reoccur, identifying and not counting the occurrence of the same flaw in the material on subsequent scans of the material, counting the flaws in a prescribed manner, etc.
Although the use of a single scanning beam has proved very successful in inspecting webs of material quickly and efficiently, the use of a single scanning beam in some respects provides limited information with respect to the type and characteristics of a particular type of flaw. Certain defects do not lend themselves to detection by a single source, for example, if the source is not of a proper wave length to be reflected, transmitted or scattered by the flaw which is to be detected. Furthermore, any size or shape information would of necessity have to be generated by several passes of the same beam over the flaw of interest with the information being stored and compared on subsequent scans which procedure adds to the uncertainty in the detection and correlation process.